Alzheimer's disease (AD), a fatal neurodegenerative disorder, afflicts close to 26.6 million people worldwide. It is characterized by accumulation in the brain of insoluble, extracellular amyloid plaques and intracellular neurofibrillary tangles considered to be, until recently, the main cause of brain dysfunction. This view has been recently reformulated to focus on soluble oligomeric Ab (oA?) aggregates, with emphasis on the role of A?42 oligomers, which are believed to cause synaptic dysfunction and eventual neuronal loss in the vulnerable regions of AD brains. Cathepsin B, a cysteine protease present in the brain, can degrade A? peptides, however, blocking its enzymatic activity in vivo with its specific inhibitor, cystatin C (CysC), rather than increasing, lowered A? peptide brain deposition in AD models. Because a portion of the CysC protein (aa101- 117), distinct from its enzyme inhibitory sequence, can bind to A? and inhibit fibril formation in vitro, we propose that generating sufficiently high levels of this peptide in vivo, might reduce A? oligomerization and neurotoxicity. We showed a) that synthetic CysC-A?-Binding Peptide (CysC-A?BP) binds specifically and preferentially to purified A?42, b) that in b AD mouse models, one with amyloid plaques and oA? (TgCRND8) and one with oA? only (APPE693Q), it co-localizes with the A? deposits in neocortex and hippocampus when incubated with brain sections in vitro, c) that similar co-localization is found 4 days after a single intranasal delivery of the peptide and d) that intranasal delivery of a CysS-GFP-expressing construct under a promoter that we have shown to be highly upregulated in the brain by several hormones produces detectable expression levels. We propose: 1. To identify conditions that generate optimal levels of CysC-A?BP in the brain. This will be done by comparing intranasal to intrathecal delivery of purified peptide and peptide-expressing construct, uncoated or encapsulated in microparticles, and to test which positive hormonal regulators of the CysS promoter optimally transactivate the CysC-A?BP construct. 2. To test whether high levels of CysC-A?BP will reduce A?-oligomer deposits in established AD and whether chronic treatment of mice before AD onset will prevent its development. Experimental conditions deemed most effective in A? oligomer reduction in response to CysC-A?BP will be utilized to test the effect this treatment has on improvement of learning and memory. By proposing to use an A?BP-fibril blocking CysC peptide, rather than the complete CysC protein, with its detrimental enzyme inhibitory activity, and by finding novel ways to deliver regulate and maximize CysC-A?BP expression in the brain, we expect to achieve beneficial effects that might be translated into therapeutic and preventive approaches to AD in humans.